1. Field of the Invention
This invention relates to the preparation of metal-exchanged highly siliceous porous crystalline materials. More specifically, this invention relates to the preparation of transition metal-exchanged zeolite catalysts.
2. Description of the Prior Art
Zeolitic materials, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline aluminosilicates. These aluminosilicates can be described as a rigid three-dimensional framework of SiO.sub.4 and AlO.sub.4 in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing aluminum is balanced by the inclusion in the crystal of a cation, for example an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic aluminosilicates. The aluminosilicates have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243), zeolite X (U.S. Pat. No. 2,882,244), zeolite Y (U.S. Pat. No. 3,130,007), zeolite ZK-5 (U.S. Pat. No. 3,247,195), zeolite ZK-4 (U.S. Pat. No. 3,314,752), zeolite ZSM-5 (U.S. Pat. No. 3,702,886), zeolite ZSM-11 (U.S. Pat. No. 3,709,979), zeolite ZSM-12 (U.S. Pat. No. 3,832,449), zeolite ZSM-20 (U.S. Pat. No. 3,972,983), ZSM-35 (U.S. Pat. No. 4,016,245), zeolites ZSM-21 and ZSM-38 (U.S. Pat. No. 4,046,859), and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO.sub.2 /Al.sub.2 O.sub.3 ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5, up to infinity. U.S. Pat. No. 3,941,871 now Re. No. 29,948, discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5 type zeolites. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe crystalline silicates or organosilicates of varying alumina and metal content.
More recently, other techniques have been developed for preparing a crystalline aluminosilicate having a high mole ratio of silica to alumina.
U.S. Pat. No. 4,112,056, for example, discloses a technique for preparing zeolites having a high silica/alumina ratio. The zeolite is crystallized from a silica-rich reaction mixture containing a source or sources of an alkali metal oxide, organic nitrogen-containing oxides, an oxide of silicon, and water. As crystallization progresses one or more sources of aluminum ions are added to maintain the concentration of aluminum ions at a steady state.
U.S. Pat. No. 3,937,791 discloses a method of removing alumina from a zeolite which comprises heating the zeolite in the presence of a cationic form of chromium in an aqueous, acidic solution of above 0.01 Normal at a pH of less than 3.5 for a time sufficient to remove the alumina. Zeolites having high silica to alumina ratios are valuable catalysts in various processes for converting hydrocarbon compounds and oxygenates such as methanol. Such processes include, for example, alkylation of aromatics with olefins, aromatization of normally gaseous olefins and paraffins, aromatization of normally liquid low molecular weight paraffins and olefins, isomerization of aromatics, paraffins and olefins, disproportionation of aromatics, transalkylation of aromatics, oligomerization of olefins and cracking and hydrocracking. All of the foregoing catalytic processes are of value since they result in upgrading of the organic charge being processed.
It is desirable in some instances to add a hydrogenation/dehydrogenation component to the zeolite of high silica-to-alumina ratio. In a common procedure, the zeolite and a solution of an exchangable metal salt have been contacted over a prolonged period of time and sometimes at elevated temperature to effect the ion exchange with metals in the zeolite structure. The amount of the hydrogenation/dehydrogenation component employed is not narrowly critical and can range from about 0.01 to about 30 weight percent based on the entire catalyst. A variety of hydrogenation components may be combined with either the zeolite and/or matrix employing well known techniques such as base exchange, impregnation, coprecipitation, cogellation, mechanical admixture of one component with the other and the like. The hydrogenation component can include metals, oxides and sulfides of metals of the transition metal groups, i.e., iron, cobalt, nickel, platinum, palladium, ruthenium, rhodium, iridium and osmium. Pre-treatment before use varies depending on the hydrogenation component present. For example, with components such as nickel-tungsten, cobalt-molybdenum, platinum and palladium, the catalyst desirably may be sulfided. With metals like platinum or palladium, a hydrogenation step may also be employed. These techniques are well known in the art and are accomplished in a conventional manner.
It has been found, however, that base exchange and impregnation processes utilizing solutions (aqueous and organic solvent) of conventional transition metal complexes when practiced on zeolites of a highly siliceous nature are less effective than when practiced on the hitherto conventional zeolites having silica to alumina mole ratios of 2 to less than about 15. On the other hand, transition metal complexes of uncommon, low ion charge or without charge, as described herein, are exceptionally effective. Although I do not wish to be bound by any theory, it is believed the increased hydrophobic nature and decreased framework aluminum density are responsible for this reduced effectiveness of base exchanging metals through the use of conventional metal complexes.